Elastomeric materials have long been prized for their ability to expand to fit over or around a larger object, and then retract to provide a snug fit around the object. This quality has been prized for centuries, and much of Europe's early exploration was in search of rubber trees for their latex.
In recent years, synthetic polymeric elastomeric materials have supplemented or replaced natural rubber. Compounds such as polyurethane rubbers, styrene block copolymers, ethylene propylene rubbers, and other synthetic polymeric elastomers are well known in the art.
Elastomeric materials can take a variety of shapes. Elastomers can be formed as threads, cords, tapes, films, fabrics, and other diverse forms. The shape and structure of the elastomeric material is guided by the intended end use of the product. For instance, elastomers are often used in garments to provide a snug fit, such as in active wear. Elastomers can also form resilient but effective barriers, such as in the cuffs of thermal garments intended to retain body heat. In these applications, the elastomer is most often in the form of threads or filaments that are incorporated into the fabric of the garment. One example of a type of garment where both fit and barrier properties are important is hygienic products such as diapers. Elastomeric materials are used in the waist, around the leg openings, and in the fasteners (for a diaper) or sides (for an underpants-type garment). The elastomeric materials in these regions improve the overall fit of the garment, and also make it much easier to both don and remove the garment. The elastomeric materials also act as resilient barriers, improving the containment capabilities of the garment while still allowing comfort and free movement to the wearer.
In a hygienic product, the elastomer can be in the form of threads, fabrics, or films. Using elastomeric threads can pose challenges in assembling the garment, since the threads must be applied as one component of many in the manufacturing process. These threads can also be weak and they tend to break, which could lead to the elastic failing even if there are redundant threads present. Elastomeric fabrics are somewhat easier to work with in a manufacturing process, but the fabrics themselves tend to be expensive both in raw materials and in the cost of manufacturing the fabric itself. Elastomeric films are easier to use in manufacturing than threads and are less expensive than elastomeric fabrics to produce. Elastomeric films also tend to be stronger than threads or fabrics, and less likely to fail in use.
However, a disadvantage of elastomeric films is that the polymers used to make the films are inherently sticky or tacky. When elastomeric films are extruded and wound into a roll, the film will tend to stick to itself or “block,” thereby becoming difficult or impossible to unwind. Blocking becomes more pronounced as the film is aged or stored in a warm environment, such as inside a storage warehouse.
The elastomeric blocking problem has been tackled in a number of ways. Antiblocking agents, which are usually powdered inorganic materials such as silica or talc, can be incorporated within the film. Antiblocking agents can also be dusted onto the outer surfaces of extruded film as the film is being formed. However, antiblocking agents must be added in large quantities to reduce blocking to an acceptable level, and these high levels of antiblock are detrimental to the elastomeric properties of the film. Another means of reducing blocking is to roughen the surface of the film, such as by embossing the film, which reduces the surface-to-surface contact of the rolled film and introduces minute air pockets that help reduce the blocking. Unfortunately, this also tends to create thinner, weaker areas of the film, which are then subject to tearing and failure when the film is stretched. Another means of reducing blocking is to incorporate a physical barrier, such as a release liner, into the roll between the layers of wound film. The release liner is then removed when the roll of film is unwound for further processing. The release liner is usually discarded, though, creating waste and a significant extra expense for the manufacturer. Yet another means of reducing elastomeric film blocking is by coextruding very thin outer layers, also called ‘skins’ or ‘capping layers,’ of an extensible or less elastomeric nonblocking polymer onto the surface of the elastomeric film. Suitable nonblocking polymers for these skins include polyolefins such as polyethylene or polypropylene. Such polyolefin skins are extensible but not elastomeric materials. They have little effect on the elastomeric properties of the film as a whole because they make up only a small fraction of the total composition of the film. However, these polyolefin skins will stretch and become irreversibly deformed when the elastomeric film as a whole is stretched or “activated” for the first time. When the stretching force on the activated elastomeric film is released, the elastomeric core will retract as it normally would. The stretched skins, which are not elastomeric, will instead wrinkle as the core retracts and create a microtextured surface.
There remains a need to effectively manufacture an elastomeric film that can be rolled and stored without blocking. Such a film should not have inferior elastomeric properties, should not create undue waste and manufacturing expense, and should present an appealing surface texture after activation.